1. Field of the Invention
The invention relates to initiators for free-radical addition polymerization, comprising as structural feature the Diels-Alder adduct of an azo group (--N.dbd.N--) with a conjugated double bond (diene).
2. Description of the Background
Initiators for free-radical addition polymerization are compounds which decompose into free radicals and so initiate the free-radical addition polymerization. The rate of decomposition is generally dependent on temperature and can be described by the Arrhenius equation. Accordingly, decomposition to free radicals begins even at relatively low temperatures and is accelerated by temperature increase.
For some fields of use, for example for the curing of powder coatings, initiators are desired which become effective only at high temperatures. Following application to the substrate surface, powder coatings must generally be heated to flowability in order to obtain a smooth and level coating film. Curing (free-radically crosslinking), which begins simultaneously by decomposition of the initiators to free radicals, leads in this case to unevennesses or defects in the coating surface. Instead, curing should not begin until after a smooth coating film has been formed by the melting of the polymer powder.
This requires initiators which decompose to free radicals only at high temperatures. The temperature dependency of the rate of decomposition in this case should be as low as possible; in other words, no decomposition below a certain temperature as complete as possible a decomposition above this temperature (non-Arrhenius behavior).